THE ROLE OF SURFACE-EXPOSED TYR-83 OF PLASTOCYANIN IN ELECTRON-TRANSFER FROM CYTOCHROME-C

The pathway of electron transfer from cytochrome c to the Cu atom of plastocyanin has been investigated using mutants of pea plastocyanin, in which the surface-exposed residue, Tyr-83, has been changed to Phe-83 or Leu-83 by site-directed mutagenesis. No significant differences in the reaction with the Tyr-83 and Phe-83 plastocyanins were observed. However, the Leu-83 mutant showed an 8-fold decrease in the rate of reduction, which was due entirely to a decrease in the intrinsic rate of electron transfer in the binary complex. This indicates that the major route for reduction of plastocyanin by cytochrome c involves Tyr-83, as has been shown previously for the natural donor, cytochrome f. The faster rates observed with cytochrome f are attributed to three factors. First, H-bonding between Tyr-83 and cytochrome f, but not cytochrome c; secondly, a slower dissociation of the reaction complex with cytochrome f; thirdly, a 20-fold faster intrinsic rate of electron transfer. Together, these factors indicate that most encounter complexes with cytochrome f are productive, whereas most with cytochrome c are unproductive.